Some of the higher tier achievements are kind of ridiculous. "Hit 1000 balloons" can be understandible, as it seems that balloons get hit the most, but 1000 spring boards and 500 mines is just too grindy. And no one knows what the Exp is for. I have gotten to the 500k mark, and have no interest in making the 1 million mark, as I hear there is no reason to.
I have a suggestion, have a number of buildings and forts that you can siege, and your chance of success depends on how many of what summons you have, and successful sieges possibly gain bonuses that could be either temporary or permanent.
The cars get really ridiculous by the time you reach that city, I mean the are almost no pedestrians, and the few that are there just get run down by the constant flow of cars.